1. Field
The present invention relates to a display element having a structure in which a plurality of display units are laminated, a method of driving the same, and an electronic paper including the same.
2. Description of the Related Art
Recently, development of electronic paper is active in enterprises, universities, and etc. As markets considered to have promising applications of electronic paper, various applied portable apparatus have been proposed, including electronic books first of all, sub-displays of mobile terminal apparatus, and display parts of IC cards. As an example of a display element used for the electronic paper, there is a liquid crystal display element that uses a liquid crystal composition having a cholesteric phase formed therein (which is referred to as cholesteric liquid crystal or chiral nematic liquid crystal and hereinafter, referred to as cholesteric liquid crystal). The cholesteric liquid crystal has excellent characteristics, for example, a semipermanent display retention characteristic (memory characteristics), a vivid color display characteristic, a high-contrast characteristic, and a high-resolution characteristic.
FIG. 19 is a cross-sectional view schematically illustrating the structure of a liquid crystal display element 51 capable of performing full color display using the cholesteric liquid crystal. The liquid crystal display element 51 has a structure in which a blue (B) display unit 46b, a green (G) display unit 46g, and a red (R) display unit 46r are laminated from a display surface in this order. In FIG. 19, the outer surface of an upper substrate 47b serves as the display surface, and external light (indicated by the arrow in a solid line) is incident on the display surface from the upper side of the substrate 47b. In addition, an observer's eye and a viewing direction (indicated by the arrow in a broken line) are schematically shown above the substrate 47b. 
The B display unit 46b includes a blue (B) liquid crystal layer 43b interposed between a pair of upper and lower substrates 47b and 49b, and a pulse voltage source 41b that applies a predetermined pulse voltage to the B liquid crystal layer 43b. The G display unit 46g includes a green (G) liquid crystal layer 43g interposed between a pair of upper and lower substrates 47g and 49g, and a pulse voltage source 41g that applies a predetermined pulse voltage to the G liquid crystal layer 43g. The R display unit 46r includes a red (R) liquid crystal layer 43r interposed between a pair of upper and lower substrates 47r and 49r, and a pulse voltage source 41r that applies a predetermined pulse voltage to the R liquid crystal layer 43r. A light absorbing layer 45 is provided on the rear surface of the lower substrate 49r of the R display unit 46r. 
The cholesteric liquid crystal used for each of the B, G, and R liquid crystal layers 43b, 43g, and 43r is a liquid crystal mixture of nematic liquid crystal and a relatively large amount of a chiral additive, for example, several tens of percent by weight of additive (which is also called a chiral material). When a relatively large amount of chiral material is added to the nematic liquid crystal, it is possible to form a cholesteric phase having nematic liquid crystal molecules strongly twisted into a helical shape.
The cholesteric liquid crystal has bistability (memory characteristics) and is possible to be in either of a planar state, a focal conic state, or an intermediate state between the planar state and the focal conic state by adjusting the strength of an electric field applied to the liquid crystal. When the cholesteric liquid crystal is in either of the planar state, the focal conic state, or the intermediate state therebetween once, the cholesteric liquid crystal stably maintains its state even when no electric field is applied.
The planar state is obtained by applying a predetermined high voltage between the upper and lower substrates 47 and 49 to apply a strong electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero. The focal conic state is obtained by applying, for example, a predetermined voltage that is lower than the high voltage between the upper and lower substrates 47 and 49 to apply an electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero.
The intermediate state between the planar state and the focal conic state is obtained by applying, for example, a voltage that is lower than that used to obtain the focal conic state between the upper and lower substrates 47 and 49 to apply an electric field to the liquid crystal layer 43 and then rapidly reducing the electric field to zero.
Next, the display principle of the liquid crystal display element 51 using the cholesteric liquid crystal will be described using the B display unit 46b as an example. FIG. 20A shows the arrangement of cholesteric liquid crystal molecules 33 in the planar state in the B liquid crystal layer 43b of the B display unit 46b. As shown in FIG. 20A, the liquid crystal molecules 33 in the planar state sequentially rotate in the thickness direction of the substrates to form a helical structure, and the helical axis of the helical structure is substantially vertical to the surfaces of the substrates.
In the planar state, light having a predetermined wavelength corresponding to the helical pitch of the liquid crystal molecules 33 is selectively reflected from the liquid crystal layer. When the average refractive index of the liquid crystal layer is n and the helical pitch is p, a wavelength λ where the highest reflectance is obtained is represented by λ=n·p.
Therefore, in order to selectively reflect blue light from the B liquid crystal layer 43b of the B display unit 46b in the planar state, the average refractive index n and the helical pitch p are determined such that, for example, the wavelength λ is 480 nm. The average refractive index n can be adjusted by selecting a liquid crystal material and a chiral material, and the helical pitch p can be adjusted by adjusting the content of the chiral material.
FIG. 20B shows the arrangement of the cholesteric liquid crystal molecules 33 in the focal conic state in the B liquid crystal layer 43b of the B display unit 46b. As shown in FIG. 20B, the liquid crystal molecules 33 in the focal conic state sequentially rotate in the in-plane direction of the substrates to form a helical structure, and the helical axis of the helical structure is substantially parallel to the surfaces of the substrates. In the focal conic state, the selectivity of the B liquid crystal layer 43b with respect to a reflection wavelength is lost, and the B liquid crystal layer 43b transmits most of incident light. The transmitted light is absorbed by the light absorbing layer 45 that is provided on the rear surface of the lower substrate 49r of the R display unit 46r whereby dark (black) display is achieved.
In the intermediate state between the planar state and the focal conic state, the ratio of the reflected light and the transmitted light is adjusted by the existential ratio of the planar state and the focal conic state, and the intensity of the reflected light varies. Therefore, it is possible to perform halftone display corresponding to the intensity of the reflected light.
As described above, it is possible to control the amount of light reflected by adjusting the alignment state of the cholesteric liquid crystal molecules 33 twisted in the helical shape. Similar to the B liquid crystal layer 43b described above, the cholesteric liquid crystal that selectively reflects green and red light in the planar state is sealed into the G liquid crystal layer 43g and the R liquid crystal layer 43r to manufacture the liquid crystal display element 51 capable of performing full color display. The liquid crystal display element 51 has memory characteristics and can perform full color display without consuming power except screen rewriting.    Patent Document 1: JP-A-10-48595
In general, it is preferable that the display element rewrites images in a short time. However, the time required for the liquid crystal display element using the cholesteric liquid crystal to perform data write scanning for screen rewriting is 10 to 100 times longer than that in a liquid crystal display element according to the related art using twisted nematic (TN) liquid crystal or super twisted nematic (STN) liquid crystal. Therefore, about 0.5 to 10 seconds are required to perform screen rewriting, and it takes a long time for an observer to recognize the content of the image displayed on the screen.